This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Primary Objective: The primary objective of this study is to determine the maximum tolerated dose (MTD) levels and recommended Phase 2 dose levels of Karenitecin when administered intravenously for 5 consecutive days repeated every 21 days concurrently administered with a fixed dose of cyclophosphamide to children with refractory or recurrent solid tumors stratified according to the following: * patients with bone marrow metastases or those who have received previous intensive myelosuppressive therapy (Stratum 1). * patients without bone marrow metastases or those who have not received previous intensive myelosuppressive therapy (Stratum 2). Secondary Objective(s): The secondary objectives of this study include the following: * To assess the toxicity associated with Karenitecin administered in combination with cyclophosphamide. * To assess the antitumor activity of Karenitecin administered in combination with cyclophosphamide.